The present invention is directed to a motor driver, and in particular a motor driver for use with induction motors.
Induction motors, and in particular fractional horsepower induction motors, are used in many household devices. For example, fractional horsepower induction motors are in widespread use in such household items as dishwashers and washing machines and in various industrial applications.
It is well known in the art to use a motor driver to control the induction motor, and in particular, the speed of the induction motor. The motor driver typically consists of a DC power supply, an inverter, and a controller for operating the inverter to generate a sinusoidal voltage waveform which is provided to the induction motor. The typical motor driver uses high frequency pulse-width modulation switch techniques to generate the sinusoidal voltage waveform.
Such drivers, however, are expensive. In the cost-competitive industry of home appliances, a few dollars difference in price can be the deciding factor in a consumer""s decision to buy one product or another. Therefore, a lower cost alternative to the typical AC driver could have a significant impact on the marketplace.
One alternative to the combination of an induction motor and an AC driver is the combination of a DC motor and a DC driver. However, the cost of the DC motor/DC driver combination is typically even more expensive than the induction motor/AC driver combination. Therefore, this combination is not the alternative which the industry is seeking.
According to an aspect of the present invention, an induction motor driver includes a DC power supply having a selectively variable voltage. The driver also includes an inverter coupled to the DC power supply and coupleable to an induction motor, the inverter selectively actuateable to generate from the selectively variable voltage of the DC power supply a low-frequency series of square-shaped pulses, the series of square-shaped pulses having a frequency and each pulse having an amplitude related to the selectively variable voltage. The driver further includes a motor driver controller coupled to the inverter and the DC power supply, the motor driver controller including a first selectively variable signal source to provide a speed signal representative of a desired motor speed, an oscillator coupled to the selectively variable signal source to generate a control signal having a frequency representative of the speed signal, an inverter controller coupled to the oscillator and the inverter to selectively actuate the inverter to vary the frequency of the series of square-shaped pulses in direct relation to the frequency of the control signal, and a power supply controller coupled to the oscillator and the DC power supply to selectively vary the voltage of the power supply in direct relation to the frequency of the control signal.